As a filtering medium for an air filter satisfying the requirement of a HEPA filter (High Efficiency Particulate Air Filter), a so-called glass filtering medium, which is a filtering medium prepared by using glass fiber, is known. The glass filtering medium of HEPA grade has high collecting efficiency of 99.97% for particles with a particle diameter of 0.3 μm, but has a high pressure loss. As a HEPA filter with low pressure loss which can be replaced with such a glass filtering medium, a filtering medium using polytetrafluoroethylene (PTFE) porous membrane, which is formed by drawing PTFE, is known. Compared to the glass filtering medium, the PTFE porous membrane has a high collecting efficiency and a low pressure loss so that the PTFE porous membrane has an excellent balance between the collecting efficiency and pressure loss.
Meanwhile, in order to have a filtering medium with long service life, it is recently required to increase a dust retention amount of a filtering medium. However, due to the thinness of a membrane, the dust retention amount of the above PTFE porous membrane is extremely small, and the PTFE porous membrane has a far less dust retention amount than that of a glass filtering medium. Accordingly, it may be considered to produce a filtering medium by attaching a melt-blown nonwoven fabric as a prefilter to a PTFE porous membrane, for example. However, in that case, because the total thickness of a filtering medium is increased due to the thickness of the nonwoven fabric, enough number of a folded filtering medium for insertion (number of pleats) cannot be ensured when the filtering medium is used for an air filter unit, and thus an increase in pressure loss, a decrease in collecting efficiency, and a decrease in dust retention amount are caused on the contrary.
Meanwhile, as a filtering medium having improved dust retention amount while maintaining the HEPA grade, there is a filtering medium for an air filter described in WO 2013/157647 A, which is suggested by the applicant of the present invention. According to the filtering medium of Patent Literature 1, it is described that, although the collecting efficiency is slightly lower than the PTFE porous membrane, since the pressure loss is greatly improved compared to the PTFE porous membrane while maintaining the HEPA grade and also the membrane thickness is high. The dust retention amount close to that of a class filtering medium can be obtained.